Simple directional coupler

ABSTRACT

Low loss high directivity wire couplers use a wire over ground transmission airline structure and a low diameter coaxial cable ending in a wire loop sensor, which is inserted into ground wall of the transmission line leading into a coupled and an isolated port. Higher, capacitively induced, electrical current, because of the confined zone between signal conductor and ground wall, compares favorably with the antiphase magnetically induced current component in the wire loop sensor and leads to increased coupling and directivity over a frequency range up to at least 70 GHz.

PRIORITY CLAIM

Not Applicable

CROSS-REFERENCE TO RELATED ARTICLES

-   -   1. Load Pull for Power Devices [online], Microwaves101        [retrieved on 2017-03-14]. Retrieved from Internet <URL:        https://www.microwaves101.com/encyclopedias/load-pull-for-power-devices>.    -   2. “Computer Controlled Microwave Tuner”, Product Note 41, Focus        Microwaves, January 1998.    -   3. Microstrip [online], Wikipedia [retrieved on 2020-04-28].        Retrieved from Internet <URL:        https://en.wikipedia.org/wiki/Microstrip>.    -   4. S-parameters [online], Microwaves101 [retrieved on        2020-04-28]. Retrieved from Internet <URL:        https://www.microwaves101.com/encyclopedias/s-parameters>.    -   5. Directional Couplers [online], Microwaves101 [retrieved on        2018-10-17]. Retrieved from Internet <URL:        http://www.microwaves101.com/encyclopedia/directionalcouplers.cfm>.    -   6. Verspecht et al., U.S. Pat. No. 7,282,926, “Method and an        apparatus for characterizing a high-frequency device-under-test        in a large signal impedance tuning environment”.    -   7. Simpson, G., U.S. Pat. No. 7,548,069, “Signal Measurement        Systems and Methods”.    -   8. Tsironis, C., U.S. Pat. No. 7,135,941, “Triple probe        automatic slide screw load pull tuner and method”.    -   9. Fourier analysis [online], Wikipedia [retrieved on        2020-04-28]. Retrieved from Internet <URL:        https://en.wikipedia.org/wiki/Fourier_analysis>    -   10. Tsironis, C., U.S. Pat. No. 9,960,472, “Programmable        amplitude and phase controller”, FIG. 9.        Introductory remark: Throughout this disclosure items on figures        start with the number of the figure, for easier reading:        Examples: item 55 is on FIG. 5; item 89 is on FIG. 8; item 112        is on FIG. 11; item 402 is on FIG. 4; item 202 is on FIG. 2 or        20 etc. . . . .

FIELD OF THE INVENTION

This invention relates to testing of microwave two-ports (transistors,DUT) using linear and non-linear measurement techniques especially undercontrolled impedances at the input and output of the transistors (loadPull measurement, see ref. 1) and also measuring and analyzing the largesignal behavior of a DUT.

BACKGROUND OF THE INVENTION

A popular method for testing and characterizing microwave transistors athigh power nonlinear operation is “load pull” and “source pull” (seeref. 1). Load pull or source pull are measurement techniques employingmicrowave tuners (see ref. 2) and other microwave test equipment. Themicrowave tuners in particular are used to manipulate the microwaveimpedance conditions under which the Device Under Test (DUT, ortransistor) is tested (FIG. 1). Bi-directional signal couplers areneeded to detect the signal waves propagating along the transmissionline towards <a> and away <b> from the DUT (FIG. 1) and to allowperforming harmonic Fourier analysis (see ref. 9) in order toreconstruct the real-time non-linear transistor response. Further-on theinstantaneous voltage-current trajectory of a transistor, typicallycalled the “load-line”, (see ref. 6) will depend on the compleximpedance presented to the transistor using harmonic tuners (see ref.8). A setup that allows this test is a “harmonic load pull setup” asshown in FIG. 1.

DESCRIPTION OF PRIOR ART

Bi-directional signal couplers have been known since long time (see ref.4); They detect forward <a> and reverse <b> travelling waves on thetransmission line and transfer the measured data to the VNA (FIG. 1). Inorder for the data to be valid the couplers must be calibrated bymeasuring their scattering (s-) parameters before (see ref. 4) andde-embed to the DUT reference plane. Typical s-parameter calibrationoccurs under 50Ω termination conditions (FIG. 7). However, when theterminations are non-50Ω, the coupling behavior, forward C and reverse I(isolation) change. The signal detected at the coupled port comes fromboth the input port and as part of the signal returning from a non-50Ωtermination (Γ2) at the output port. In load pull operations inparticular the tuners create (on purpose) non-50Ω test conditions. Therelation describing this phenomenon is:C(Γ2)=S31+S32*S21*Γ2/(1−Γ2*S22)≈S31+S32*Γ2  {eq. 1}andI(Γ2)=S41+S42*S21*Γ2/(1−Γ2*S22)≈S41+S42*Γ2  {eq. 2}whereby Γ2 is the reflection created by the tuner at port 2 and C(Γ2)and I(Γ2) are the new values of the transmission S31 and isolation S41between port 1 and ports 3 and 4 or the ratio of signal power detectedat ports 3 and 4 divided by the injected signal power into port 1 (FIG.7). The Directivity is a coupler property defined as S31/S41=S42/S32,depending which port, 1 or 2, is defined as the input port. If Γ2=0 thenC(0)=S31 and I(0)=S41, as follows from {eq.1, 2}. The importantquantities are S32 and Γ2, that is the isolation and the load reflectionfactor; since |S21|≈1, and |S22|≈0 it is, finally, the product Γ2*S32that determines the sensitivity of the coupling factor on the mismatchcreated by the tuner. An ideal directivity coupler should therefore havea Directivity of infinite, or S41=S32=0. This not being possible,“Directivity” is a key and distinguishing performance of any directionalcoupler, especially when used in a non-50Ω, i.e. |Γ2|>0 test environmentas shown in FIGS. 1 and 7. Commercially available compact widebandcouplers (see ref. 5) have Directivity values between 10 and 20 dB. Thecoupler presented here is simpler to make, is extremely wideband andexceeds this level of Directivity.

BRIEF DESCRIPTION OF THE INVENTION

Signal couplers are in general bi-directional. When the isolated port(FIG. 7) is terminated with characteristic impedance, i.e., no signal isreflected back into the coupler, it is called a directional coupler,because the larger portion of the detected signal comes from onedirection (forward). Of course, because the couplers are bi-directional,in reverse direction the isolated port becomes the coupled port and thecoupled port becomes the isolated port. The couplers dealt with in thisdisclosure are bi-directional couplers. The directional coupler of thepresent invention (FIGS. 6A and 6B) uses the wire-over-ground (WOG)transmission line structure and the advantages offered by thisembodiment are: a) the simplicity of the transmission airline (oneground-wall instead of two) offering the benefit of relaxed parallelismrequirements, which are mandatory in a slabline (FIG. 2), and b) thestronger concentration of electric field in the zone between signalconductor and closer-by ground surface (FIG. 5), which leads to highercapacitive coupling and induced electric currents; such electricallyinduced currents, when added to the magnetically induced currents (FIG.4), increase the forward coupling by roughly 16 dB (FIGS. 9A and 9B),comparison between prior art IV-probe, FIG. 3, wave-probe, FIG. 2, ref.6, shown as NEW versus TOP) and greater Directivity by 15 dB.Directivity data of prior art IV couplers (see ref. 7) are known to beinferior. The coupler is made by inserting a U-shaped electro-magneticwire loop sensor into a hole in the ground-wall at the level of thesignal conductor and connecting its branches to coaxial cables leadingto the coupled and isolated coaxial ports.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The invention and its mode of operation will be more clearly understoodfrom the following detailed description when read with the appendeddrawings in which:

FIG. 1 depicts prior art a Load pull test setup for measuring powercontours and real time incident and reflected waves and load reflectionfactor of a DUT, using bi-directional coupler and network analyzer.

FIG. 2 depicts prior art, signal coupler of type “wave-probe”.

FIG. 3 depicts prior art, a voltage-current (I-V) coupler.

FIG. 4 depicts prior art, magnetically induced and capacitively coupledcurrents inside the coupling loop of a wire coupler.

FIG. 5 depicts prior art, electric and magnetic field distribution in atransmission line using the “wire over ground, WOG” concept, and the“distance to ground over wire diameter” ratio yielding characteristicimpedance Zo=50 Ohms.

FIG. 6A through 6B depict a wire over ground (WOG) high coupling anddirectivity coupler; FIG. 6A depicts a cross section at the wire looplevel; FIG. 6B depicts a top view.

FIG. 7 depicts prior art, definition of transmission, reflection andcoupling RF parameters in a directional coupler.

FIG. 8 depicts prior art, comparison of coupling and isolation (i.e.Directivity) of coaxial and slabline based wire couplers.

FIG. 9A through 9B depict partly prior art i.e. all figures and tracesnot marked NEW: FIG. 9A depicts schematically the compared embodiments;FIG. 9B depicts comparison of coupling and isolation coefficient ofthree wire-coupler configurations from 2 to 68 GHz: NEW is the couplerof this invention (WOG, wire over ground), SIDE is a configuration wherethe wire loop sensor is inserted perpendicular to the slabline wall andTOP is the configuration where the wire loop sensor is inserted from thetop into the slot of the slabline, as in prior art of FIG. 2.

FIG. 10 depicts partly prior art, return loss of wire couplerconfigurations as in FIG. 9A.

FIG. 11A through 11B depict characteristics of the embodiment of thepresent invention: FIG. 11A depicts coupling and isolation; FIG. 11Bdepicts return loss.

FIG. 12 depicts top view of detailed structure of the directionalcoupler of the present invention

DETAILED DESCRIPTION OF THE INVENTION

The simple directional coupler uses a low loss wire-over-ground (WOG)transmission airline which is popular mostly in form of a dielectricsubstrate based microstrip configuration (see ref. 3). The advantagesoffered by this method are twofold: a) it is mechanically simpler than acoaxial or a parallel-plate airline (slabline); b) it offers a strongerelectric field concentration in the zone between signal conductor andground wall. The simplicity of the WOG transmission airline allows arelaxed parallelism requirements; the stronger concentration of electricfield in the zone between signal conductor and closer-by ground plan(FIG. 5) leads to higher induced electric currents, in the wire loopsensor, which increases the coupled signal and decreases the isolatedsignal, thus increasing the coupling and directivity at the same time.Knowing that too-low coupling factors and limited directivity are theweaknesses of such wire couplers, this solution is twofold beneficial.

The coupling and isolation mechanism, first described in ref. 6 works asfollows (FIG. 4): the RF signal current Is inside the signal conductor(40) creates a magnetic field H around it. This pulsing magnetic field H(42) couples into the parallel running wire loop sensor (41-43) andcreates a magnetically induced current I_(H) which flows from branch(43) through the bottom of the “U” shaped loop (44) into branch (41).Since the bottom of the wire loop sensor runs parallel to the signalconductor (40) there is a capacitive coupling between the two. Thiscapacitive coupling induces, capacitive current I_(E) into either 50 Ohmterminated branches (41) and (43). These currents are proportional tothe electric field in this region. Inside branch (43) the magneticallyinduced current I_(H) and the electric one I_(E) add yielding a totalcurrent I_(H)+I_(E). Inside branch (41) these currents run antiphase andsubtract. The total signal power in the load to branch (43) is therefore|I_(E)+I_(H)|²*Zo and in branch (41) |I_(H)−I_(E)|²*Zo. This createsboth the forward coupling into branch (43) and the isolation in branch(41).

Since the predominant coupling mechanism is magnetic I_(H) is alwayslarger than I_(E). Or, if we can increase I_(E) the differenceI_(H)−I_(E) in branch (41) tends towards zero. This increases isolationand directivity. At the same time, it also increases I_(H)+I_(E); thisincreases forward coupling. The objective is therefore to increaseI_(E).

This is achieved using the WOG structure, where, due to the higherproximity between the signal conductor and the ground the electric field(and the capacitive coupling with the wire loop sensor) are strongestamong the various structures studied so far (FIGS. 8 and 9). Forcharacteristic impedance Zo=50 Ohms, in a cylindrical coaxial structurethe ratio gap G between the center conductor (50) with diameter D—andexternal mantle (51) is G/D=0.651. In a slabline structure the ratio isG/D=0.406. And in a WOG structure (FIG. 5) it is G/D=0.152. Thisindicates that capacitive coupling and electric field is strongest inthe WOG structure. The strength of the capacitive coupling increases bythe inverse of the corresponding G/D. The prior art embodiment of FIG.2, is, in this respect, an outsider, both regarding strong coupling andhigh directivity, since the wire loop sensor is inserted in the regionof the weakest electric and weak magnetic fields.

In FIG. 8, a comparison is shown between the original slabline-basedstructure (FIG. 2) with a coaxial structure, where the cylindricalcenter conductor traverses a cylindrical tube, as shown in FIG. 9A. Thefact that the wire loop sensor in FIG. 2 is placed in free space abovethe center conductor in the region of low electric field, shows in lowcoupling and directivity, compared with the coaxial structure where boththe electric and magnetic fields are homogenous and stronger.

FIG. 9B compares the coupling and directivity of three non-cylindricalcoaxial structures: TOP coupler is the original slabline based structureof FIG. 2; it shows the weakest performance, both in coupling anddirectivity, because the wire loop sensor is placed in an region of weakelectric and magnetic field. Main reason for this embodiment is theabsence of modifications to the airline and main advantage is thepossibility to move the wire loop sensor along the slabline to controlthe coupling phase, whatever this may be beneficial for (see ref. 10).SIDE coupler is a slabline based structure, where the wire loop sensoris inserted into a hole in one side-wall as close to the centerconductor as possible. Here the electric field is approximately 5×stronger than in the space above the center conductor, leading to higherelectric coupling yielding 13 dB higher forward coupling and alsoapproximately 13 dB higher directivity. NEW coupler is the WOG-basedembodiment, where the wire loop sensor is inserted into a perpendicularhole in the ground wall (FIGS. 6A, 6B and 12), yielding the highestelectric coupling and leading to the highest coupling factor (18 dB morethan FIG. 2 and 5 dB more than the side coupler) and approximately 3 dBhigher directivity than the side coupler and 16 dB higher directivitythan FIG. 2. Compared with the cylindrical coaxial embodiment of FIG. 8the new coupler is in par concerning directivity (obviously because inthe coaxial case both the magnetic and electric currents are equallyweaker) but exceeds the cylindrical coaxial embodiment in forwardcoupling by at least 12 dB (−22 dB versus −10 dB).

The protrusion of the wire coupler into the high field area of thecavity of the transmission line (FIGS. 6A, 6B and 12) is a reason ofconcern regarding the residual return loss of the coupler. FIG. 10 showsa comparison of three of the embodiments of FIG. 9A. It is clear thatthe results do not differ significantly. In all cases the residualreturn loss does not exceed approximately 30 dB, which, realistically,is acceptable for any practical application.

In conclusion the new WOG (wire over ground) embodiment is superior incoupling to all alternative embodiments and superior in directivity toall, except the coaxial structure, where they are equal. Obviousalternatives and modifications to the herein disclosed general conceptof the use of a WOG transmission line for making a wideband highcoupling and directivity wire coupler shall not impede in the validityof the invention.

What is claimed is:
 1. A directional RF signal coupler having an inputport, an output port, a coupled port and an isolated port, saiddirectional RF signal coupler comprising: a) a wire-over-groundtransmission airline between the input and output ports, saidwire-over-ground transmission airline comprising a metallic ground walland a signal conductor between the input and output ports, and b) a “U”shaped electro-magnetic wire loop sensor having a bottom section and twobranches; wherein the “U” shaped electro-magnetic wire loop sensor isinserted in a hole of the metallic ground wall and protrudes into anarea between the metallic ground wall and the signal conductor with thebottom section running parallel to the signal conductor, and whereineach branch of the “U” shaped electro-magnetic wire loop sensor extendsinto a center conductor of a coaxial cable terminating into either thecoupled or the isolated port.
 2. The directional RF signal coupler ofclaim 1, wherein the “U” shaped wire loop sensor is inserted in a zone,where the signal conductor is closest to the ground wall.
 3. Thedirectional RF signal coupler of claim 1, wherein the signal conductoris cylindrical in shape.
 4. The directional RF signal coupler of claim1, wherein a characteristic impedance of wire-over-ground transmissionairline is 50 Ohms.